Whenever you play a guitar or other stringed instrument, you leave something of yourself behind. We’re not talking about artistic sacrifice or heartfelt emotion, we’re talking about actual physical traces of yourself in the form of sweat, body oils, tiny bits of dead skin, and any dirt or soils that were on your hands when you sat down to jam. Unless you have a high-powered microscope, you can’t see these materials on your strings, but they’re rubbing off and accumulating between the windings every time you play.